1. Field of the Invention
The present invention relates to a method of operating a punch press during start-up and stopping operations. In particular the punch press includes a positionable drive; a coupling/brake device having a coupling and a braking unit including structural members which move during the operation of the punch press; moving structural members including an eccentric shaft driven by a positionable drive via the coupling unit of the coupling/ brake device; at least one push rod supported on the eccentric shaft; a ram pivotably mounted to the at least one push rod to be driven by the eccentric shaft; an upper tool including at least one working tool, the upper tool being pivotably mounted to the at least one push rod; a stationary punch press table; and at least one lower tool mounted to the punch press table; so that the upper and lower tools work on a strip-like workpiece located therebetween. The eccentric shaft of the punch press, due to the total inertia moment of the moving structural members of the punch press and due to the driving torque transmitted by the coupling unit of the coupling/brake device, rotates during their acceleration from a stand-still state to the state of rotating at the operationally rated number of strokes through start-up angles to a predetermined operation angle where a first contact between the at least one working tool and the strip-like workpiece occurs.
2. Description of the Prior Art
It is a generally known fact that in high-speed punch presses the known increasing of the forces of inertia of the accelerating structural members pose a large problem at an increasing rotational speed or number of strokes, resp., which problem has specifically an influence onto the quality and preciseness of products produced by such high-speed punch presses. Due to the accelerations and decelerations of the predominantly oscillating structural members and due to the counterforces produced during the working of a respective workpiece the punch press and specifically its moving parts suffer elastic deformations and displacements occur, furthermore, in the various bearings, which have quite a negative influence on the preciseness at the produced products.
The precision of a punched product depends among others strongly from the respective height position of the ram. As is generally known to the persons skilled in the prevailing art, this height position of the ram determines or sets, respectively, the closed tool height position or the depth of penetration at the operating of the machine.
If a fixed height position of the ram is set for predetermined or given, respectively, stamping, embossing and cutting operations at a given number of strokes, it is a commonly known fact that e.g. the embossments do not reach the rated or designed, respectively, depth when the punch press operates at a relatively low number of strokes and conversely, the embossings have a too large depth or the depth of penetration of the punching tools, respectively, is too large at a higher number of strokes which latter condition leads, as is generally known, to undesirably large wear at the corresponding tool members.
A variety of procedures for a controlling of the height position of the ram or depth of penetration, respectively, based on the number of strokes, have become known. Reference is made here for instance to the Swiss patent specification CH-A-676 445.
During a punching operation, and specifically if such operation proceeds by means of multiple press tools, there is the desire that no waste is produced also during the start-up and stopping of the punch press due to the above mentioned forces of inertia and specifically in case of high-speed punch presses the start-up and stopping operation causes considerable troubles in this respect.
when starting a punch press up, initially the drive, generally an electric motor is energized and run up to the rated operational speed, i.e. the rated operational number of strokes of the punch press. For the start-up proper of the punch press, its eccentric shaft is coupled by a closing of the coupling to the drive motor rotating already at a rated operational speed and specifically to the fly-wheel and accordingly accelerated from the state of stand-still up to the rated rotational speed. The dynamic behavior of the punch press during the first or initial working operation, e.g. the first punching operation, is thereby defined first punch behavior or first impact behavior of the punch press and it is this first punch behavior which determines the quality of the first punched product of a running series of punched products made during the operating of the punch press.
At the known start-up methods of high-speed punch presses, this first punch behavior is now such that the first punching operation is not made at the rated operating number of strokes such as during the next subsequent, then continuous punching operations, i.e. the dynamic behavior of the punch press during the first stroke is quite different from the subsequent strokes which has a quite negative effect regarding the precision of the first part produced.
A reduction of the operating number of strokes could obviously eliminate this drawback, but then the production is decreased, i.e. the number of products produced within a given time span is lower.